The invention is generally directed to an electronic timepiece which provide for continuous hand movement and in particular to an analog electronic timepiece which provides continuous and smooth hand movement from a discrete timepiece movement.
Reference is made to FIG. 1 wherein a timepiece, generally indicated as 100, constructed in accordance with Japanese Patent Publication No. 56-47512 is shown. Timepiece 100, which is only shown in partial relevant section, includes a drive train 101, shown schematically, power gear 102 and power pinion 109 supported by pivots 105 and 119 between main plate 117 and gear train bridge 123. Pivot 119 is supported in pivot support 121 on main plate 117, which is secured in place by screw 112.
Pinion 109 drives third gear 122 which is supported by pivots 106. Third pinion 110, which is fixed to third gear 122, in turn drives minute hand gear 108. To produce a continuous motion of the second hand or sweep second hand, power pinion 109 drives driving magnet 115. Driving magnet 115 is supported by magnet support 114, which is directly coupled to power pinion 109. A first following magnet 116 is enclosed by a viscous fluid 113, thereby producing viscous resistance to rotation. First following magnet 116 and viscous fluid 113 are supported by support plate 120 on main plate 117. First following magnet 116 is driven by the attractive force of driving magnet 115. A second following magnet 118 is driven by first following magnet 116. Second following magnet 118 is coupled to second hand display shaft 111. In this way, the conventional analog electronic timepiece with a discrete time keeping movement provides for continous hand movement.
In this type of conventional timepiece, the portion of the timepiece which stores the rotary energy and the portion which gradually releases the rotary energy is formed as a single member, i.e. the driving magnet and following magnets. This results in several problems. If the size or configuration of the following magnet is changed to vary the amount of energy which can be stored, the viscous resistance to the viscous fluid tends to change, thereby resulting in jerky, non-smooth hand movement. On the other hand, if the size of the gap between the following magnet and main plate is changed, the magnitude of the magnetic attractive force tends to vary.
Variations in the phase deviation, i.e. the angle between the driving magnet and the first following magnet on the one hand and the angle between the first following magnet and the second following magnet on the other hand, cause the attractive or repulsive forces along the axial direction of the magnets to change. This results in the magnets, and particularly the first following magnet shown in FIG. 1, moving upward or downward within the limits of the clearance providing by the viscous fluid. The movement of the first following magnet within the cavity of viscous fluid changes the viscous resistance to movement of the first following magnet. It also changes the orientation of the magnet which causes changes in friction due to the thrust force and the direct friction of the edges of the first following magnet against the main plate. A non-uniformity of rotation is the result.
In addition, the conventional timepiece of the type shown in FIG. 1 has a multiple step structure, which makes it difficult to reduce the thickness of the timepiece, and also results in there not being enough of a span between the bearings. The axes for supporting the hand are unstably supported and the hand tends to become undesirably tilted during operation.
Another problem with the conventional structure is the sine wave relationship between the magnetic attractive force and the rotational angle. As a result, when the angle between the driving magnet, and the first following magnet or the first following magnet and the second following magnet is greater than 90.degree., the magnitude of the restoring force is reduced for increased angular deviation and the magnet system does not properly function to control the rotation of the following magnet. When the angle between the magnets is about either 0.degree. or 90.degree., the restoring force barely changes as the angle between the magnets changes so that responsive speed control is not achieved. This is seen by examination of a sine curve at 0.degree. or 90.degree. where the rate of change in amplitude per change in angle is small. As a result, when the angle between the magnets is at 0.degree. or 90.degree., due to fluctuations or changes in the viscous load, the speed is not effectively controlled.
In a real world situation there are many forces which result in fluctuations in the magnet attractive force and viscous load or fluctuations due to the dimensional accuracy and uneven magnetization. These stresses to the system may result in angles between the driving magnet and the following magnet being occasionally greater than 180.degree.. In these situations the following magnet not only fails to correctly control the speed, but it in fact rotates in the opposite direction. Thus an inappropriate time is displayed.
Further, because the magnets are rotated in the timepiece, there is magnetic interference betweent the magnets and the stepping motor. As a result, the layout of the components is severely restricted. In addition, viscous fluids having a very high viscosity is required to obtain the high viscous resistance at low rotary speed required by the conventional arrangement which requires as many rotary magnet assemblies as there are hands. This increases the cost and difficulty of assembling the timepiece.
Accordingly, there is a need for an improved timepiece which converts the discrete movement of the timekeeping circuitry and step motor to continuous movement of the hand which is reliable, effective, relatively insensitive to internal and external stresses, compact, easy to repair and adaptable to a thin timepiece.